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Fan Affinity Law — Airflow vs Speed

Also known as fan laws · VFD airflow change

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}

Worked example: 8000 cfm at 900 rpm re-sheaved to 1080 rpm → 9600 cfm = 271841.7 L/min — press Try an example to run it live, then adjust anything.

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Fan Affinity Law — Airflow vs Speed explained

N2N1Q1Q2

Air is a fluid like any other, so a fan wheel obeys the same first affinity law as a pump impeller: 20% more rpm buys 20% more cfm. A balancer measuring 8000 cfm at 900 rpm who needs design airflow speeds the wheel to 1080 rpm and reads 9600 cfm. On belt-driven units the adjustment is made not with a dial but with sheaves — swapping the motor pulley for one 20% larger raises fan rpm by 20%, which is why an air-balance report always records both sheave sizes.

The universal field mistake is treating this as free. Airflow is linear but brake horsepower is cubic, so the 20% re-sheave that fixes the airflow deficiency raises fan power by 73% and will happily trip an already fully loaded motor. Check the motor nameplate amps before you cut the belt, not after.

Fan Affinity Law — Airflow vs Speed formula

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}
Where
  • Q1Q_1= Airflow at speed 1 (L/min)
  • N1N_1= Fan speed 1 (rpm)
  • Q2Q_2= Airflow at speed 2 (L/min)
  • N2N_2= Fan speed 2 (rpm)

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Part of this formula set

The pump and fan affinity laws

7 formulasHow flow, head and power scale with impeller speed and diameter — the first, second and third power relationships behind every VFD retrofit.